Given the unreliability of differential diagnosis within the Pervasive Developmental Disorders (PDD), and the heterogeneity of children with subsyndrome diagnoses such as infantile autism (IA), an important step in furthering biological research into these disorder is to elucidate the behavioral syndromes. Abnormalities of social behavior are a hallmark of IA and other PDD syndromes, but have received very little systematic study. Although these abnormalities are often viewed as secondary to cognitive deficits, it is suggested that they represent areas of primary pathology in some PDD children. We therefore propose to study social deficits, and their relationships to certain cognitive functions, in IA and childhood onset PDD children, comparing them to MA-matched normal and retarded children, and investigating behavioral subgroups within the PDD group. We propose to administer (1) a set of experimental tasks to measure comprehension of expressions of affect in others, (2) a videotaped play session for direct observation of social responsiveness, (3) a neuropsychological battery, and (4) family, developmental, and current behavior checklists. From our previous work, it is suggested that (1) PDD children will perform more poorly on all proposed social measures than MA-matched controls, and that within the PDD group, (2) measures of cognitive level and social responsiveness will be independent, (3) ability to perform elicited facial mimicry will be correlated with MA while spontaneous mimicry will be correlated with social responsiveness, (4) some PDD children will fail to orient to both neutral auditory and social stimuli, while some will show selective failure to orient to social stimuli, (5) chldren in different cognitive profile subgroups will not differ in social responsiveness, and (6) children who show more profound social unresponsiveness will also show a cluster of qualitative abnormalities on neuropsychological test performance and teacher reports of everyday behavior, including preoccupations, rituals, fragmentation, segmentation, losing set, perseveration, and long attention span for preferred activities.